...but i bet it will cause problem with other programs that need libpng15...

Yes it will, i build it some months ago 'cause is a dependency for the last mupen64plus, but then at reboot windows decorations and icons where not drawn anymore, maybe it was because i did something wrong. But if i remember well i barely edited the slackbuild of the libpng package already installed by default.

No, I have tried the symlink, and it doesn't work. It DID however, break some other symlinks .

As far as I know of, the only thing depending on this version of libpng1.2 is the gmic_gimp plug-in. It also needs libfftw, libfftw_threads. I am not in contact with GREYC's and do not know if they're working on updates to suit the newer versions of libpng.